Referring to FIG. 1, the geographic area serviced by a wireless telecommunications system 100 is divided into a plurality of spatially distinct areas called "cells." For ease of analysis each cell 102, 104, 106 is typically approximated and schematically represented by a hexagon in a honeycomb pattern, however, each cell is actually of an irregular shape that depends on the topography of the terrain surrounding the cell. Each cell 102, 104, 106 contains one base station 112, 114, 116, respectively. Each of base stations 112, 114, 116 includes equipment to communicate with Mobile Switching Center ("MSC") 120, which is connected to local and/or long-distance transmission network 122, such as a public switch telephone network (PSTN). Each base station 112, 114, 116 also includes radios and antennas that the base station uses to communicate over an air interface with wireless terminals 124, 126. The air interface may be an air interface for digital signals such as TDMA, GSM, or CDMA, or it may be an analog air interface.
One example of a time slotted air interface is a time division multiple access system, such as North American TDMA or GSM systems. In a Frequency Division Duplex (FDD) system using a time slotted air interface, the radio spectrum is divided into two bands. Half the spectrum is used for uplink, the transmission of signals from the mobile unit the base station, and half the spectrum is used for downlink, the transmission of signals from the base station to the mobile unit. The two bands are further broken down into individual channels. For TDMA each channel is 30 kHz, and for GSM each channel is 200 kHz.
In typical North American TDMA system, each time slot has a duration of 6.67 milli seconds (ms), six time slots comprise a frame. Each time slot includes a preamble, an information message, and tail bits. The time slots cycle after every third time slot. An uplink signal from a mobile unit is only received by the base station during the user's designated time slots, and a downlink signal from the base station to a mobile unit is only transmitted during the user's designated time slots. For example, signals from user one are in the first time slot, signals from user two are in the following time slot, signals from user three are in the third time slot.
Separate transmit and receive antennas are often used at the base station to transmit and receive signals over the air interface, although a common single antenna and a means of separating the transmitted signal from the received signal can also be used. The receive antennas at the base station can be omni-directional antennas that cover the entire cell or directional antennas that cover one sector of one of the cells. Cells 102, 104, 106 can be sectored into six 60.degree. geographical sectors each having directional antennas that cover the individual sectors or three 120.degree. geographical sectors each having directional antenna that cover the individual sectors.
The size of cells 102, 104, 106 in wireless communication system 100 is determined by a number of factors, including the gain of the base station's receive antenna, the gain of the mobile terminals' antennas, the transmitter power of the base station, the transmitter power of the mobile terminals, the base station's receiver sensitivity, the mobile unit's receiver sensitivity, and height of the base stations antenna(s). In many wireless communication systems, the limiting factor on cell size is the uplink range, which is dependent on the transmitter power of the mobile terminal. Under most circumstances, base stations 112, 114, 116 have enough power to increase the downlink range by increasing the transmitter power if signals are too weak to reliably reach users at the outermost edge of the sectors. However, if the uplink signals from the mobile terminal are too weak to reliably reach the base station, the mobile terminal may not have the capacity to increase its transmission power, limiting the useable range, and therefore the size of the cell to the distance from which the signal from the mobile terminal can reliably reach the base station.
Smaller cell size requires additional cells in order to be able to provide coverage over the entire area serviced by wireless telecommunications system 100. Additional cells require the purchase, installation and maintenance of more equipment, as well as increased requirements and costs of site acquisition for the base stations, interconnection facilities, and system support.
Higher gain narrower beam receive antennas at each base station reduce interference by eliminating reception of signals outside the sub-sector. Higher gain narrower beam transmit antennas reduce interference since signals transmitted outside the sub-sector are not received at full strength within the sub-sector.
Higher gain receive antennas at each base station also permit reliable reception of uplink signals at greater distances. Because gain is inversely related to antenna beamwidth, higher gain is possible by the use of narrow beamwidth antennas providing coverage over only a portion of a sector. One proposal for implementing a system with narrow beamwidth antennas providing coverage over only a portion of a sector is to use a multi-beam antenna with a plurality of antenna beams that are narrower than the sector and collectively cover an entire sector. The signals are received on the antenna beams, and one antenna beam is selected to be connected to the receiver based on the amplitude and/or signal to spurious signal ratio of the signal on that antenna beam. The systems continues monitoring the sector and when the amplitude and/or signal to spurious signal ratio of the uplink signal received on another antenna beam is larger than the current antenna connected to the receiver, the system switches to the better performing antenna beam.
A problem with such a system is that when the system switches the antennas connected to the receiver there is a discontinuity in the signal. In an analog system, the discontinuity in the signal is manifested as a loud noise, or as it is known in the industry a pop. This loud noise can last as long as a second and is uncomfortable for the user. The noise both disturbs the user's conversation, making it a distraction and annoyance, and it is physically uncomfortable to the user, since it is loud and unpleasant.
In a digital system, the discontinuity in the signal results in bit error rates, and the loss of synchronization, and a disturbance of the automatic gain control (AGC). This is typically manifested in either a noise or a muting of the signal. Both of which are a distraction and annoyance to the user.